1. Technical Field
The present invention relates to printed circuit boards and, particularly, to a method of making a flexible printed circuit board.
2. Description of Related Art
Flexible printed circuit boards (FPCs) are in widespread use in electrical devices, such as cellular phones, digital still cameras, notebooks and so on, due to advantages of lightness, thinness, and flexibility. An FPC generally comprises at least one metal foil layer and at least one insulating layer with an optional adhesive layer between the metal foil layer and the insulating layer. Copper is the most popular material for use in the metal foil layer and is etched to form the circuit. Polyimide (PI) is widely used as the material of the insulating layer.
Nowadays, FPCs only having one metal foil layer are gradually being replaced by FPCs having more than one laminated metal foil layer in the course of pursuing greater miniaturization of electrical devices. To electrically connect the metal foil layers of an FPC, at least one through hole or via is defined through two metal foil layers of the FPC, and plated along an inner wall thereof with a conductive material. Commonly at least one of the outermost layers of an FPC is a metal foil layer, and the plating of the through holes results in that the outermost metal foil layer is also coated with an extra layer of plating thereon. The extra layer of plating makes the FPC less flexible.
What is needed, therefore, is a method of making an FPC which can avoid a formation of an extra layer of plating on the outermost metal foil layer.